The most common source of an A7 error code is when a unit with an internal pre-heater turns off the fans because the temperature has dropped below the safe operating limit. This is exactly what we want the unit to do in these conditions so that the HRV or ERV core doesn' t freeze. The pre-heater works to avoid or delay a necessary shut-down, but if the temperature gets cold enough even the pre-heater isn' t enough to protect the core, so the fans will be turned off. Because this is the desired function, the A7 isn' t so much an error code as it is a notification. The unit is programmed to periodically check the air temperatures and resume normal operation when it' s safe to run. But even after the fans are running again the A7 notification will remain on the controller to alert the user that there has been a shut-down. To clear the code, simply unplug the HRV/ERV for a minute or so and plug it back in.
There may be other, less-common reasons for an A7 code, so if the code persists be sure to contact your Zehnder service technician.